The radial velocity method

The radial velocity method of detecting exoplanets looks for tiny backwards and forwards motion of the central star, due to the changing direction of the gravitational pull from an (unseen) orbiting exoplanet. If the star is moving towards us its spectrum is blueshifted, and redshifted correspondingly when it moves away from us. By looking for regular shifts in the spectrum of the star — and so measuring any velocity changes — any periodic effects due to the influence of a companion can be seen.